Radical polymerizations usually are started by initiators such as peroxides, hydroperoxides or azo compounds. The rate at which the initiator breaks down into radicals depends on the temperature. Conventional radical initiators are stable at room temperature. Depending on their structure, their decomposition into radicals generally is triggered in the temperature range of 60.degree. C., which starts the polymerization. However, a polymerization at low temperatures is also frequently desired. The decomposition of peroxides at room temperature can be obtained with suitable redox systems, for example peroxide/amine. Two components must be metered for the polymerization with this in mind.
It is well-known that boroalkyl compounds can initiate radical polymerizations by themselves at room temperature. In this case the boron compounds are aerobically active initiators, which means that they develop their activity in the presence of oxygen, usually air. The oxygen needed for the activation is practically omnipresent and need not be added separately. Preferred for use are simply trialkyl boron compounds, e.g. triethyl boron or tri-n-butyl boron. The use of trialkyl boron compounds as polymerization initiators is described, e.g., in the U.S. Pat. Nos. 3,476,727; 3,633,490, 2,985,633; and GB-PS No. 1,113,722. In the U.S. Pat. No. 4,167,616 reaction products of butadiene and diborane are described as polymerization starters and respective polymeric reaction products are also described. However, these polymeric boroalkyl compounds are characterized by the fact that at times short segments of C--C bonds are interrupted by, or are linked by B--C units.
Among the essential advantages of the boroalkyl initiators are: The polymerizations take place also at low temperature, the starter system is in single-component form and, finally, the rate of polymerization can be changed by varying the oxygen available.
The simple trialkyl boron compounds, for example triethyl boron or tri-n-butyl boron, but also reaction products of dienes, e.g. butadiene (1,3), pentadiene, (1,3) and others, with diborane have ignition temperatures below 0.degree. C. and ignite spontaneously very readily, please refer to DE-OS No. 23 21 215, for example. They present considerable safety risk during handling, even with extreme precautions. Oxidation to esters of boric acid, which do not initiate polymerizations, occurs as oxygen becomes available. The use and especially the metering of boroalkyls as well as their production process thus require the complete exclusion of oxygen. Actually, the required amount of substance must be stored under inert gas in a completely air tight container. Any seepage of oxygen into the storage container must be prevented.